Implement the exception return consistency checks
described in the v7M pseudocode ExceptionReturn().
Inspired by a patch from Michael Davidsaver's series, but
this is a reimplementation from scratch based on the
ARM ARM pseudocode.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>